Not going to hurt to leave it plugged in for a few days. Unplugging it will like you have been doing will give a much longer life to the lift, its not the power its the fact that you disconnect the ground when you unplug it. If you have poor grounding at the pole (neutral) then your house current will find its way down your lift and cause it to rot. I also use huge zincs on mine. Using the key does not disconnect everything from the lift.
Mark, what problems are you having? I've have 3 hi-tide lifts (1-4post and 2 elevators) all 14 years old, never a problem with any of them. Is it the lift or the dealer in your area?